Methods and apparatuses for path selection in a packet network

ABSTRACT

Embodiments including methods, systems, and apparatuses for distributing, processing, and reacting to path information distributed via a service-agnostic packet fabric for the purpose of enabling path selection are disclosed. By configuring two ingress line cards to send path quality words to each other via the switch fabric, compare the path quality words, and determine whether to transmit traffic to an egress line card via the switch fabric based on the comparison of the path quality words, the embodiments enable a central switch fabric to be unaware of the paths that it carries, and enable both ingress and egress bandwidth of the switch fabric to be sized according to the facilities for which it is terminating. The switch fabric does not need to support working and protection paths simultaneously in some embodiments, allowing it to be scaled appropriately to termination facilities.

BACKGROUND OF THE INVENTION

Path and line selection for protection in a network, such as, forexample, a Synchronous Optical Networking (SONET)/Synchronous DigitalHierarchy (SDH) network, has historically been accomplished viaselection in a central switch fabric or data ingress or egress of a portcard.

Historically, switch fabrics have been built with specific technologyfor the signals it is expected to switch. Depending on the approach,many Time-Division Multiplexing (TDM) switch fabrics employ protectionselection at an ingress, central switch fabric, or egress, whichinvolves building bandwidth-scaled switch fabrics.

Ingress selection typically relies on a fixed physical relationshipbetween line cards and the exchanging of TDM signals external to thecentral fabric. Egress protection selection provide flexible protectionassociations; however, they require building fabrics with extra capacityto support switching of at least two paths to an egress line card.Central fabric selection is a versatile approach; however, for fastswitch-over times, the fabric must be built with protocol awareness andthe ability to detect and react to protection switches quickly.

For generic packet-based switches carrying TDM signals, the centralswitch fabric is unaware of the services that it carries and is unableto make any decisions as to the selection of a protected data path.

SUMMARY OF THE INVENTION

Sharing of line and path information between ingress line cards enablesa protection selection to be made between two or more peer line cards atthe ingress to a packet switch fabric. Switch fabric multicastcapability can be used to exchange protected line and path informationand enables a flexible relationship between line cards with protectedpaths. This limits the bandwidth required by the switch fabric becausepath selection is performed prior to switching.

An example embodiment of the present invention is a switching domainthat includes a switch fabric and first and second ingress line cards.Each line card sends quality words to each other via the switch fabric.The line cards then compare the quality words and determine whether totransmit traffic to a third line card via the switch fabric based on thecomparison.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing will be apparent from the following more particulardescription of example embodiments of the invention, as illustrated inthe accompanying drawings in which like reference characters refer tothe same parts throughout the different views. The drawings are notnecessarily to scale, emphasis instead being placed upon illustratingembodiments of the present invention.

FIG. 1 is a block diagram illustrating a switching domain including twoprotected ingress paths switched to an unprotected egress path. Thefigure only shows data flow in one direction.

FIG. 2 is a block diagram illustrating a switching domain including twoprotected ingress paths switched to two protected egress paths. Thefigure only shows data flow in one direction.

FIG. 3A is a block diagram illustrating a line card processing aningress TDM path and including a message generator and a comparator.

FIG. 3B is a detailed block diagram illustrating a line card processingan ingress TDM path and including a message generator and a comparator.

FIG. 4A is a flow diagram illustrating an embodiment of a procedure linecards can use to determine whether to transmit traffic based on acomparison of path quality words.

FIG. 4B is a detailed flow diagram illustrating an embodiment of aprocedure line cards can use to determine whether to transmit trafficbased on a comparison of path quality words.

FIG. 5 is a block diagram illustrating a switch fabric that includes anaddress table and a replicator.

FIG. 6 is a flow diagram illustrating an embodiment of a procedure aswitch fabric can use to replicate path quality words.

FIG. 7 is a block diagram illustrating a line card that includes anaddress table and an associator.

FIG. 8 is a flow diagram illustrating an embodiment of a procedure linecards can use to generate a message packet and transmit the messagepacket to a switch fabric.

FIG. 9 is a block diagram illustrating a line card that includes anaddress table, comparison module, and transmission module.

FIG. 10 is a flow diagram illustrating an embodiment of a procedure aline card can use to cause traffic to be transmitted based on acomparison of path quality words.

DETAILED DESCRIPTION OF THE INVENTION

A description of example embodiments of the invention follows.

Disclosed embodiments include methods, systems, and apparatuses fordistributing, processing, and reacting to path information distributedvia a service-agnostic packet fabric for the purpose of enabling pathselection. Embodiments may serve as a protected data path selectionmechanisms for, for example, SONET/SDH or Optical Transport Network(OTN) TDM signals adapted to packets and switched by a packet switchfabric. The embodiments enable establishment of a messaging structureand processing between modules carrying working and protection paths,and enable path protection selection at a path ingress module bycontrolling which ingress data stream is transmitted to an egress pathvia the packet switch fabric. In some embodiments, the egress pathmodule is unaware of the processing behind the ingress path selection.The embodiments enable use of a central switch fabric that is unaware ofthe paths that it carries. Both ingress and egress bandwidth of theswitch fabric are sized according to the facilities for which itterminates. The switch fabric does not need to support extra bandwidthfor switching working and protection paths simultaneously.

FIG. 1 is a block diagram illustrating a switching domain 100 thatincludes two protected ingress paths 101 a,b switched to an unprotectedegress path 102, according to an example embodiment of the presentinvention. The switching domain 100 includes a switch fabric 105 andfirst and second ingress line cards 110, 115. Each line card 110, 115 isconfigured to send path quality words (PQWs) or messages (PQMs)including one or more PQWs 125, 130 to each other via the switch fabric105. In some embodiments, PQMs may be referred to as Path Quality Words(PQWs) where each message includes one path quality word (PQW). In otherembodiments, each PQM may include one or more PQWs. As an example, ifthere were 192 source addresses and 192 corresponding PQWs, a given PQMmay include 24 PQWs, and 8 PQMs would be used for the 192 PQWs. Inembodiments where the PQMs include multiple PQWs, each of the line cardmay parse the PQMs and extract the PQWs relevant to the particular linecard. As used herein, the terms PQW and PQM may be used interchangeably.

Each of the line cards 110, 115 compares the PQWs 125, 130 anddetermines whether to transmit traffic 135, 140 to a third (e.g.,egress) line card 120 via the switch fabric 105 based on the comparisonof the PQWs 125, 130. In another embodiment, only one of the line cards110, 115 compares the PQWs 125, 130 and notifies the other one of theline cards 110, 115 of the result in some manner, such as in the form ofa status notification or command of what the other line card is to do,such as transmit or not transmit its received traffic 135, 140.

The PQWs 125, 130 may represent the quality of received ingress traffic135, 140 over, for example, a protected path or line 101 a,b. Eachingress line card 110, 115 may multicast its PQW 125, 130 to all otherline cards in the switching domain 100, and, in some embodiments, onlyone of the two line cards transmits traffic for a protected path, whereeach line card (e.g., 110) determines independently whether to transmittraffic (e.g., 135) based on whether the PQW it sent (e.g., 125)represents a higher path quality than the PQW (e.g., 130) sent by theother of the first and second line cards (e.g., 115). Additionally, eachline card 110, 115 may transmit traffic 135, 140 for a protected path ifa PQW is not received from the other of the first and second line cardswithin a predetermined interval. The third (e.g., egress) line card 120may accept traffic from either ingress line card 110, 115.

Example embodiments may have the following components: (1) definition ofa path quality word (PQW) to be distributed through the packet fabric,(2) distribution of PQWs using, for example, multicast addressing, (3)ingress PQW processing, and (4) egress processing.

The meaning of the fields of a PQW, or PQM that may include one or morePQWs, may relate to hardware sensed path defects. They may also includesoftware-based fields for dynamic communications between path endpoints, and include source addressing of path or line terminations. Insome embodiments, a PQW is created at each path ingress (e.g., at twoingress line cards), is distributed by the fabric, and is mutuallymonitored at the path ingress modules (e.g., at the two ingress linecards). The PQW may include a source address based on physical facility,an alarm priority based on a detected path or line defect, asoftware-defined field, a transmit enable indicator, or a messagevalidity indicator.

A source address may denote a shelf, slot, facility number, and pathidentifier, and may be used to identify incoming PQWs to process. Analarm priority may be a number corresponding to the severity of a signaldefect or in-band forced protection switching, where higher severitiesmay correspond to higher alarm priority numbers, in which case the pathwith the lower alarm priority may be selected as the selected path. Forexample, a “loss of signal” may have priority level 3, “alarm indicationsignal” (AIS) may have priority level 2, “signal degrade” or “forcedprotection switch” may have priority level 1, and “no defect” may havepriority level 0. A software-defined field may be populated by anapplication and used to communicate information between path ingress andegress points. For example, software-defined bits may be used toinitiate ingress path selection manually. Ties may be decided based on apredetermined algorithm. A “transmit enable” indicator may indicatewhether the source address is transmitting traffic to the path egress,and a “message validity” indicator may indicate the “sanity” of themessage, in which case any messages without a message validity indicatorwill not be processed.

The distribution of PQWs may be by multicast addressing and may includebatching of PQWs into a single PQM. As described above, PQWs may bebatched together in a single packet and distributed to all switchingmodules using a single multicast address. Each packet may have anormalized batch address indicating a relative path identifierconsisting of, for example, a shelf, slot, and path identifier. Thenormalized path identifier may be used for filtering PQWs received fromremote sources for protected paths. Further, the PQWs may have a periodof, for example, 1 to 5 milliseconds to facilitate path protectionswitches in less than 50 milliseconds.

Ingress PQW processing may involve mutual path selection betweenprotected sources by implementing an automatic squelching mechanism forthe deselected path to enable quicker and unsynchronized path selectionchanges. Ingress PQW processing may involve creating a PQW anddistributing it via a multicast address, monitoring for PQWs from aremote working/protection path, reacting to changing PQWs from theremote path, and selecting a path over which to transmit traffic to anegress module.

In some embodiments, each line card associated with a protected ingresspath monitors PQWs generated for the local path and compares them toreceived PQWs for a remote path. The remote path may be identified byits source address. Thus, for each ingress path, there may be two PQWsto monitor: a local and remote PQW. For each ingress path, the PQW'salarm severity, for example, may be monitored, and the path with thelower severity is enabled for transmission to the path egress point. Ifboth paths have the same severity level, then a path designated byprovisioning or arbitration, for example, may be selected fortransmission to the egress path.

Selection may depend on working/protection designation, “hold off', and“wait to restore” configurations. Additionally, a “watchdog” functionmay be used to select a protection path when there is a working pathfailure. Each line card may perform the watchdog function and begintransmitting traffic to the egress line card upon detecting a loss ofPQWs from an associated working/protection path or detection of aselected source ceasing to transmit via a PQW or PQM “transmit enabled”field. If the selected ingress path changes by any of the abovemechanisms, the “working” path transmission may be automaticallysquelched, and the transmit enabled indication may be cleared. In thiscase, the “protecting” line card may also determine that it is then thebetter path source and begin transmitting traffic to the path egresswithout coordinating with the original “working” path line card, thus,enabling quicker protection switches.

Egress processing may include multiple-source-address lookup to enablereceiving traffic from either the working or protection path. Sourceaddressing may be used when associating an egress path with an ingresspath. For example, the processing for each egress path may be programmedwith two or more possible source addresses; thus, when path selection isperformed at the ingress, there is no additional programming required atthe path egress.

Continuing to refer to FIG. 1, FIG. 1 illustrates an example of usingPQWs for cross-connecting protected paths on Line Card 1 110 and LineCard 2 115 to a single egress path on Line Card 3 120. FIG. 1illustrates a protected-to-unprotected cross-connection. For each path101 a,b switched through the switch fabric 105, PQWs 125, 130 areexchanged between Line Cards 1 and 2 110, 115 for the protected ingresspaths 101 a,b. These source line cards 110, 115 compare their local PQW(e.g., 125) with a received remote PQW (e.g., 130) and choose the pathwith the best quality based on, for example, an alarm severity.

In the illustrated example, Line Card 1 110 has the selected ingresspath and transmits a data stream 135 to the egress path on Line Card 3120. The egress Line Card 3 120 is capable of receiving data from eitherLine Card 1 or 2 110, 115. As described above, Line Cards 1 and 2 110,115 may use a “transmit enable” indicator and automatic squelching(e.g., disabling an output) to coordinate which line card transmitstraffic to path egress Line Card 3 120. The switch fabric 105 maymulticast PQWs 125, 130 from a source line card to all other line cards,although the PQWs 125, 130 may only be processed by the ingress linecards 110, 115 and ignored by other line cards.

In the example embodiment of FIG. 1, only one ingress line card (125 or130) transmits traffic to the egress line card 120. No path selectionoccurs in the switch fabric 105 or at the egress line card 120. When theegress path 102 is unprotected, as in FIG. 1, a unicast address may beused through the packet fabric 105 to transmit the traffic 135, 140.When the egress path is protected, as in FIG. 2, a multicast address maybe used to connect a single selected ingress path to multiple egresspaths. Note that this multicasting may be in addition to optionalmulticasting of the PQWs shared between or among ingress line cards 110,115.

FIG. 2 is a block diagram illustrating a switching domain 200 similar tothat of FIG. 1, but including two protected ingress paths 201 a,bswitched to two protected egress paths 202 a,b, according to an exampleembodiment of the present invention. The switching domain 200 of FIG. 2illustrates an example of using PQWs 225, 230 for cross-connectingprotected paths 201 a,b on Line Cards 1 and 2 210, 215 to protectedpaths 202 a,b on Line Cards 3 and 4 220, 245. It should be appreciatedthat FIGS. 1 and 2 illustrate a unidirectional example, and that otherembodiments may implement path selection in both directions. For theembodiment of FIG. 2, PQW processing and exchange between the source andprotected paths are as described above for FIG. 1, but in FIG. 2, thepacket fabric 205 uses multicast addresses to transmit traffic 235, 240through the packet fabric 205. Thus, an ingress line card 210, 215transmits its traffic 235, 240 to multiple egress line cards 220, 245.As in the example embodiment described above for FIG. 1, only one linecard acts as a data source for the egress paths. Line Cards 3 and 4 220,245 are both configured to receive traffic from either Line Cards 1 or 2210, 215.

FIG. 3A is a block diagram 300 illustrating a line card 310 processingan ingress TDM path and including a message generator 305 and acomparator 315, according to an example embodiment of the presentinvention. According to the example embodiment, the line card 310 is aningress line card that includes a word generator 305 that generates alocal PQW 325 and sends the local PQW 325 to a remote ingress line cardvia a switch fabric. In at least one embodiment, if the ingress linecards are located close enough, direct communications (independent of aswitch fabric) can alternatively be done. The ingress line card 310 alsoincludes a comparator 315 that compares the local PQW 325 to a remotePQW 327 received from the remote ingress line card and that determineswhether to transmit traffic 335 based on a comparison of the local andremote PQWs 325, 327. The line card 310 may also include a selector thatenables or disables the transmission of traffic 335 based on thedetermination made by the comparator 315.

The local and remote PQWs 325, 327 may represent the quality of receivedingress traffic at the ingress line cards, respectively. Further, theingress line card 310 may multicast the local PQW 325 to all other linecards in operative communication with the switch fabric. The ingressline card 310 may then determine independently whether to transmittraffic 335 based on whether the local PQW 325 represents a higher pathquality than the remote PQW 327. In the absence of a remote PQW 327,e.g., if the remote PQW 327 is not received from the remote line card,for example, within a predetermined interval, the ingress line card 310may transmit traffic 335 for its protected path. A description of themulticasting procedure is presented below in reference to FIGS. 5 and 6.

FIG. 3B is a detailed block diagram 301, similar to FIG. 3A,illustrating a line card 311 processing an ingress TDM path andincluding a PQW generator 306 and a comparator/selector 316, accordingto an example embodiment of the present invention. According to theexample embodiment, PQW generator 306 generates a local PQW 326 andsends the local PQW 326 to a remote ingress line card via a switchfabric. On the way to the switch fabric, the local PQW 326 may be packedinto a PQM 337 by a PQM packer 308 for efficiency. Thecomparator/selector 316 compares the local PQW 326 to a remote PQW 338received from the remote ingress line card and that determines whetherto transmit traffic 336 based on a comparison of the local and remotePQWs 326, 338. The remote PQW 338 may be extracted from a remote PQM 328by a PQM associator 309. The selector enables or disables thetransmission of traffic 336 based on the determination made by thecomparator.

FIG. 4A is a flow diagram 400 illustrating how line cards, such as theingress line card 310 of FIG. 3, can determine whether to transmittraffic based on a comparison of PQWs, according to an exampleembodiment of the present invention. According to the exampleembodiment, a method 400 of an ingress line card's switching trafficincludes generating a local PQW (405) and sending the local PQW to aremote ingress line card via a switch fabric (410). The method furtherincludes comparing the local PQW to a remote PQW received from theremote ingress line card (415) and then determining whether to transmittraffic to the switch fabric based on the comparison of the local andremote PQWs (420).

As in the above embodiments, the local PQW may represent the quality ofreceived ingress traffic, sending the local PQW (410) may includemulticasting or broadcasting the local PQW to all other line cards inoperative communication with the switch fabric, determining whether totransmit traffic (420) may include independently determining whether totransmit traffic based on whether the local PQW represents a higher pathquality than the remote PQW, and determining whether to transmit traffic(420) may include transmitting traffic if the remote PQW is not receivedfrom the remote line card within a predetermined interval, for example.Additionally, after the determination, the line card's traffic may besquelched if it is determined that it is not to transmit traffic.

FIG. 4B is a detailed flow diagram 401, similar to FIG. 4A, alsoillustrating how line cards, such as the ingress line card 310 of FIG.3, can determine whether to transmit traffic based on a comparison ofPQWs, according to an example embodiment of the present invention.According to the example embodiment, a method 401 of an ingress linecard's switching traffic includes generating a local PQW and insertingit into a local PQM (406) and sending the local PQM to a remote ingressline card via a switch fabric (411). The method further includescomparing the local PQW to a remote PQW received from the remote ingressline card and extracted from a remote PQM (416, 421) and thendetermining whether to transmit traffic to the switch fabric based onthe comparison of the local and remote PQWs (426) or using a “watchdog”function, as described above.

FIG. 5 is a block diagram 500 illustrating a switch fabric 505 includingan address table 510 and a replicator 515, according to an exampleembodiment of the present invention. According to the exampleembodiment, the switch fabric 505 includes a table 510 with entriesrepresenting addresses of line cards and a replicator 515 thatreplicates packets 525 containing PQWs with respective associated sourceaddresses. The replicator 515 transmits the replicated packets 528 a,bto line cards according to the entries in the table 510. The switchfabric 505 need not be aware of the PQWs. In some embodiments, the table510 may be a multicast table, in which case, the replicator 515 maymulticast the replicated packets 528 a,b to line cards according to theentries in the multicast table 510. Further, at least some of thepackets may include multiple parts that each include a source addressand a PQW. The multicasting may be performed in a standard manner ofmulticasting or may be performed in a customized manner.

Multicasting PQWs has an advantage over multiple independent messagingsbecause of reduced overhead traffic. In this case, one PQW per ingressline card can be sent into the switch fabric 505, and multiple PQWs canbe transmitted to the ingress line cards. For example, in the case ofeight ingress line cards, eight local PQWs can be sent to the switchfabric 505 (i.e., one for each ingress line card), and sixty-four PQWscan be sent out from the switch fabric 505 to the ingress line cards.This is a reduction from seven to one PQWs in the upstream directionfrom each line card at an expense of just one more PQW in the downstreamdirection to each line card, for a net bandwidth savings. To furtherincrease bandwidth savings, multiple PQWs can be batched into a PQM. Thereplicator 575 may perform multicasting in a manner described in U.S.Pat. No. 7,639,685, the entire teachings of which are incorporatedherein by reference in their entirely.

FIG. 6 is a flow diagram 600 illustrating replication of PQWs in aswitch fabric, according to an example embodiment of the presentinvention. According to the example embodiment, the flow diagram 600includes configuring a table in the switch fabric, such as the table 510in FIG. 5, with entries representing addresses of line cards (605). Theswitch fabric (e.g., switch fabric 505) then replicates packetscontaining PQWs with respective associated source addresses (610) andtransmits the replicated packets to the line cards according to theentries in the table (615).

FIG. 7 is a block diagram 700 illustrating a line card 710 including anaddress table 705 and an associator 715, according to an exampleembodiment of the present invention. According to the exampleembodiment, the line card 710 includes a table 705 with entriesrepresenting source addresses of local ingress paths on which localtraffic can be received, and an associator 715 that generates a messagepacket 725 with an entry and a PQW. The PQW may be based on line andpath alarms 712. The entry of the message packet 725 is based on thesource addresses entries of the table 705 and, in some embodiments, maybe a multicast address. Additionally, the associator 715 transmits themessage packet 725 into a line card message stream 735 to be transmittedto a switch fabric, where the switch fabric is known to be configured toinspect the message packet 725 and direct the message packet 725 to aline card corresponding to the entry in the message packet. In someembodiments, the message packets may include multiple parts that eachinclude a PQW and are packed together using a PQM packer 726. In someembodiments, the line card 710 may include a defect monitor, whichmonitors for actual or user-created defects on which to base thecreation of PQWs.

FIG. 8 is a flow diagram 800 illustrating a method by which a line card,such as the line card 710 of FIG. 7, can generate a message packet(e.g., PQM) and transmit the message packet to a switch fabric,according to an example embodiment of the present invention. Accordingto the example embodiment, the flow diagram 800 generates messagepackets by configuring a table with entries representing sourceaddresses of local protected paths on which local traffic can bereceived (805). The line card then generates message packets with anentry (e.g., PQW) (810), and thereafter transmits the PQW to a switchfabric configured to inspect the message packet, which, in turn, directsthe message packet to a line card corresponding to the entry in themessage packet (815). The line card may generate the PQW using a tablethat includes a list of defects and how the defects should be denoted inthe PQW.

FIG. 9 is a block diagram 900 illustrating a line card 910 including anaddress table 905, comparison module 915, and transmission module 920,according to an example embodiment of the present invention. Accordingto the example embodiment, the line card 910 includes the table 905,with entries representing source addresses of local and remoteprotection paths, and the comparison module 915, which compares PQWs(e.g., local and remote PQWs 912, 913) representing qualities of localand remote protected paths. The comparison module 915 causes a localtraffic transmission module 920 to enable or disable transmission ofreceived local traffic 935 based on the comparison and the entries ofthe table 905.

FIG. 10 is a flow diagram 1000 illustrating how a line card, such as theline card 910 of FIG. 9, causes traffic to be transmitted based on acomparison of PQWs, according to an example embodiment of the presentinvention. According to the example embodiment, the flow diagram 1000enables and disables traffic transmission by configuring a table withentries representing source addresses of local and remote protectionpaths (1005). PQWs representing qualities of local and remote protectedpaths are then compared (1010), and transmission of received localtraffic is enabled or disabled based on the comparison and the entriesof the table (1015). In some embodiments, the PQWs may represent defectscreated by users to facilitate manual protection switches. In someembodiments, a “watchdog” function may be used to determine whether theline card is to transmit traffic. Additionally, after the determination,the line card's traffic may be squelched if it is determined that it isnot to transmit traffic.

While this invention has been particularly shown and described withreferences to example embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the scope of the inventionencompassed by the appended claims. It should be understood that theflow diagrams of FIGS. 4A, 4B, 6, 8, and 10 are examples that caninclude more or fewer components, be partitioned into subunits, or beimplemented in different combinations. Moreover, the flow diagrams maybe implemented, for example, in hardware, firmware, field programmablegate arrays (FPGAs), or software. If implemented in software, thesoftware may be written in any software language suitable for use inswitching domains, switch fabrics, and line cards as illustrated inFIGS. 1-3B, 5, 7, and 9. The software may be embodied on any form ofnon-transient computer readable medium, such as RAM, ROM, or magnetic oroptical disk, and loaded and executed by generic or custom processor(s).

Further, while embodiments have been illustrated in the context ofnetwork elements, including line cards and switch fabrics, it should beunderstood that the example embodiments can also be applied to a networkat a macro-level, such as between networks using differentcommunications protocols and an inter-working node there-between, whereingress networks may employ the inter-working node to allow each ingressnetwork to determine which is providing traffic with the better quality(or other metric) such that two or more ingress networks can provide one(or more) streams of traffic to an egress network via the inter-workingnode. Similarly, the example embodiments can be employed at amicro-level, such as in a handheld device having multiple input-outputdevices (e.g., chips) in operative communication with each other via anarbitrator or switching device. For purposes of this description and thefollowing claims, networks, line cards, electronic devices, andinter-working nodes, switch fabrics, and electronic devices areequivalent other then their respective scales.

1. A switching domain, comprising: a switch fabric; and first and secondingress line cards configured to (i) send path quality words to eachother via the switch fabric, (ii) compare the path quality words, and(iii) determine whether to transmit traffic to a third line card via theswitch fabric based on the comparison of the path quality words.
 2. Aswitching domain as in claim 1 wherein the path quality words representquality of received ingress traffic.
 3. A switching domain as in claim 1wherein the first and second ingress line cards are further configuredto multicast their respective path quality words to all line cards inthe switching domain.
 4. A switching domain as in claim 1 wherein theswitching domain is configured to have either of the first or secondline cards transmit traffic for a protected path, and wherein the firstand second line cards are each further configured to determineindependently whether to transmit traffic based on whether the pathquality word it sent represents a higher path quality than the pathquality word sent by the other of the first and second line cards.
 5. Aswitching domain as in claim 1 wherein the first and second line cardsare each further configured to transmit traffic for a protected path ifa path quality word is not received from the other of the first andsecond line cards within a predetermined interval.
 6. A switching domainas in claim 1 wherein the first and second ingress line cards are eachconfigured to squelch its traffic based on the determination of whetherto transmit traffic.
 7. A switching domain as in claim 1 wherein thethird line card is configured to accept traffic from the first or secondingress line cards.
 8. A switching domain as in claim 1 wherein thefirst and second ingress line cards are further configured to batchmultiple path quality words together before sending the batched pathquality words to each other via the switch fabric.
 9. A switching domainas in claim 8 wherein the first and second ingress line cards arefurther configured to extract path quality words from a messageincluding multiple path quality words.
 10. An ingress line card in aswitching domain, the ingress line card comprising: a message generatorconfigured to generate a local path quality word and to send the localpath quality word to a remote ingress line card via a switch fabric; anda comparator configured to compare the local path quality word to aremote path quality word received from the remote ingress line card andto determine whether to transmit traffic based on a comparison of thelocal and remote path quality words.
 11. An ingress line card as inclaim 10 wherein the local path quality word represents the quality ofreceived ingress traffic at the ingress line card for a given protectedpath, and the remote path quality word represents the quality ofreceived ingress traffic at the remote ingress line card for the givenprotected path.
 12. An ingress line card as in claim 10 wherein theingress line card is configured to multicast the local path quality wordto all line cards in operative communication with the switch fabric. 13.An ingress line card as in claim 10 wherein the ingress line card isconfigured to determine independently whether to transmit traffic for aprotected path based on whether the local path quality word represents ahigher path quality than the remote path quality word.
 14. An ingressline card as in claim 10 wherein the ingress line card is configured totransmit traffic for a protected path if the remote path quality word isnot received from the remote line card within a predetermined interval.15. An ingress line card as in claim 10 wherein the message generator isfurther configured to generate a message with multiple local pathquality words to be sent to the remote ingress line card.
 16. An ingressline card as in claim 15 wherein the comparator is further configured toextract path quality words from a message including multiple pathquality words.
 17. A method of switching traffic, the method comprising:generating a local path quality word; sending the local path qualityword to a remote ingress line card via a switch fabric; comparing thelocal path quality word to a remote path quality word received from theremote ingress line card; and determining whether to transmit trafficbased on the comparison of the local and remote path quality words. 18.A method as in claim 17 wherein generating a local path quality wordincludes generating a local path quality word representing the qualityof received ingress traffic.
 19. A method as in claim 17 wherein sendingthe local path quality word includes multicasting the local path qualityword to all line cards in operative communication with the switchfabric.
 20. A method as in claim 17 wherein determining whether totransmit traffic includes independently determining whether to transmittraffic based on whether the local path quality word represents a higherpath quality than the remote path quality word.
 21. A method as in claim17 wherein determining whether to transmit traffic includes transmittingtraffic if the remote path quality word is not received from the remoteline card within a predetermined interval.
 22. A method as in claim 17wherein determining whether to transmit traffic includes squelchingtraffic.
 23. A method as in claim 17 wherein sending the local pathquality word includes batching the local path quality word together withother local path quality word before sending.
 24. A method as in claim23 wherein comparing the local and remote path quality words includesextracting the remote path quality word from a message including otherremote path quality words.
 25. A switch fabric, comprising: a tableconfigured with entries representing addresses of line cards; and areplicator configured to replicate packets containing path quality wordswith respective source addresses and to transmit the replicated packetsto the line cards according to the entries in the table.
 26. A switchfabric as in claim 25 wherein (i) the table is multicast table and (ii)the replicator is further configured to multicast the replicated packetsto the line cards according to the entries in the multicast table.
 27. Aswitch fabric as in claim 25 wherein at least some packets includemultiple parts, each part including a source address and one or morepath quality word.
 28. A line card in a switching domain, comprising: atable having entries representing source addresses of local ingresspaths on which local traffic can be received; and an associatorconfigured to generate, based on the table, a message packet with anentry and a path quality word, and configured to transmit the messagepacket into a line card message stream to be transmitted to a switchfabric known to be configured to inspect the message packet and directthe message packet to at least one other line card corresponding to theentry in the message packet.
 29. A line card as in claim 28 wherein theentry of the message packet is a multicast address.
 30. A line card asin claim 28 wherein message packets include multiple parts, each partincluding a source address and a path quality word.
 31. A line card,comprising: a table configured with entries representing sourceaddresses of local and remote protection paths; and a comparison moduleconfigured to compare path quality words representing qualities of localand remote protected paths and configured to cause a local traffictransmission module to enable or disable transmission of received localtraffic based on the comparison and the entries of the table.